Gocode is a helper tool which is intended to be integraded with your source code editor, like vim and emacs. It provides several advanced capabilities, which currently includes:

Context-sensitive autocompletion
It is called daemon, because it uses client/server architecture for caching purposes. In particular, it makes autocompletions very fast. Typical autocompletion time with warm cache is 30ms, which is barely noticeable.

Main project site: https://github.com/nsf/gocode